A chemical process called methylation caused by smoking can deactivate or alter how gene work. After five years of staying smoke-free, most of the damage is undone - but some of it can stick for decades.

Many of the genes affected are tied to heart disease and cancer.

Equally important is our finding that even after someone stops smoking, we still see the effects of smoking on their DNA.

Dr. Stephanie London, study author

Overall, more than 7,000 genes -- one-third of the human genome -- can be affected by methylation. Some of those genes had never been linked to smoking-related diseases before.

Those genes could be used as markers to see who is at risk for smoking-related diseases in the future.

So... is quitting worth it?

Yes. Very much so, according to Dr. Stephanie London and her team from the National Institute of Environmental Health Sciences. But risks still remain.

Fortunately, smoking rates have fallen drastically in the United States. Only about 15 percent of American adults and 11 percent of high school students smoke now, NBC News reports.

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